Sages,

So relating to a previous post:

https://www.brickboard.com/RWD/volvo/1636869/220/240/260/280/fresh_air_vent_always_closed_regardless_button_position.html

I've had a "hissss" out of my passenger side foot well when I shut the car off.

So my question(s) are a follows:
1. What causes this?
2. Will this effect my gas mileage?
3. What effect does this have on the engine performance?

I'm asking as I just took the car on part of a long distance shake down cruise after completely tons of repairs and upgrades and I was expecting to get better than 25 MPG on the first part of the trip. Currently I'm attributing the poor mileage to heading over all the mountains in western Md, and West Virginia, and to the fact that I was driving the posted speed limit of 70 mph.

I'm also asking this as my new me purchase of a 90' makes the same hissing sound when you shut it off, though all the vents appear to be operational, though I have not personally checked each one yet.

As always, thanks in advance to any and all information posted here.

Matt